The present invention relates to a device for fixating optical fibres in an optical fibre ribbon cable when cutting the fibres in said cable.
The same principle applied when cutting glass is also applied in present-day optical fibre cutting machines, where a shallow groove or score line is made in the surface of the glass and the glass then broken off along the score line. Cutting machines may be constructed in three different ways, as described below:
Alternative a). An optofibre is clamped between two clamps and a fracture line is then made on the fibre transversely to the fibre direction, wherewith the fibre breaks off as a result of the mechanical tension present in the fibre.
Alternative b). An optofibre is placed on a hard supporting surface in the absence of any tension. A fracture line, or weakening, is then made on the fibre transversely to the fibre direction and a specific pressure is applied to said fracture line or weakening, so as to break off the fibre.
Alternative c). A knife is placed on an optofibre transversely to its fibre direction, wherewith the fibre is severed by pressing the knife thereagainst.
Any one of these methods can be used in the construction of an optofibre cutting machine. Each method has advantages and disadvantages that must be balanced against one another when constructing a new cutting machine. Optic fibre ribbon cable cutting machines are almost always constructed in accordance with alternative b). When cutting optical fibres in the form of optic fibre ribbon cables, it is important that the length of the separated fibres do not differ from one another to any great extent, and that the fibre cutting angle is not over excessive.
U.S. Pat. No. 5,106,006 describes a method of fixating and cutting the fibres of a fibre ribbon cable, where the fibres are placed between two supports. These supports consist of two parts and the fibres are placed between respective parts. Each part includes a rubber surface which is placed against the fibres so as to hold the fibres in a fixed vertical and lateral position. Mounted in the space between the supports is a knife that acts on the fibres to make a fracture line or weakening. A weight is then dropped down onto the fibres and a force applied at the fracture line. The length and the angle of each individual fibre will depend on the configuration or appearance of the fracture line; depth and position Since the knife is located at a predetermined level in relation to the optofibres, the fibres must be positioned at one and the same level each time a cut is made. The aforedescribed method will function satisfactorily provided that the rubber surfaces are not worn and that they still have their initial resilience. If the rubber surfaces become excessively worn, the optofibres will be positioned at slightly different levels.
The plastic jacket around the fibre is normally stripped by a method referred to as hot jacket removing. This involves heating the plastic jacket and then cutting the jacket with the aid of two knives whilst pulling the jacket from the fibres at the same time. Because the plastic jacket is deformed as it is drawn out by the knives, the exposed fibres will not be parallel with one another, but will be splayed in all directions. The fibres will thus present different angles to the knife in the cutting machine, and will therefore be cut at mutually different lengths and cutting angles. The lengths of respective fibres will thus vary slightly, since the device used includes no means for guiding the fibres automatically and therewith bring the fibres perpendicular to the knife during cutting of the fibres.
U.S. Pat. No. 5,312,468 teaches a method in which a fibre ribbon cable is placed between two supports and the supports then displaced relative to each other. By following this procedure prior to making a fracture line or weakening with the aid of a knife, it is possible to obtain a specific cutting angle. Fixation of the fibres in respective supports can be effected with the aid of v-grooves, among other things.
The problem with present-day cutting machines is that the difference in length between severed fibres in an optic fibre ribbon cable is too great and that the cutting angle can vary. This makes satisfactory splicing of two fibre ribbon cables difficult to achieve. The result is often a poor splice, or join, with particularly uneven splice attenuating values.
The present invention addresses a problem as to how a fracture line in each individual fibre of an optic fibre ribbon cable can be given mutually the same depth, by fixating the fibres vertically.
Another problem addressed by the invention is one of positioning the fibres of an optic fibre ribbon cable parallel with one another, so that a right angle will be obtained between each individual fibre and a cutting tool that has made said fracture line.
An object of the present invention is thus to provide conditions which will enable fibres in an optic fibre ribbon cable to be cut so that each individual fibre will have the same length as the other fibres and present the smallest possible cutting angle.
Subsequent to exposing the fibres of a fibre ribbon cable, the cable is placed in a fixture or jig, which is preferably self-centering so that the ribbon cable will always be centred around a specific centre line. The jig is mounted on the cutting machine in a predetermined position, by placing the jig in a machine-mounted holder and pressed against one side of said holder. The fibres of the ribbon cable are placed between two supports. These supports consist of two parts and the fibres are placed between respective parts. The bottom part, which lies proximal to the jig, has a non-resilient surface that includes V-shaped recesses or grooves, whilst the upper part includes a resilient surface. The other support consists of two similar parts, including a resilient surface.
The jig is positioned so that the fibres will be located in the grooves on the first support and on the resilient surface on the second support. The fibres can be positioned linearly in the grooves, despite possibly being deformed during the process of stripping the plastic covering, by angling the jig in relation to the surface containing said grooves. The upper parts of said supports press the fibres down into the V-grooves, therewith fixing the fibres both vertically and laterally. The fibres will also be held in mutual parallel relationship in the space between said supports. A fracture line, or weakening, is made on the fibres with the aid of a knife mounted in the space between said supports. A weight is then dropped down onto the f fibres so as to apply a force at the fracture line.
One advantage afforded by the invention is that the fibres can be caused to extend linearly in relation to one another automatically, by placing the jig in a predetermined position and introducing an angle between the grooved surface and said jig.
The invention will now be described in more detail with reference to preferred embodiments thereof and also with reference to the accompanying drawings.